1. Field of the Invention
The present invention relates to a process for the production, in industrial quantities, of an elastomeric film, particularly useful as an intermediate layer for safety laminated glass panels, said film being obtained as a composite continuous ribbon, interposed between two protective films, by means of a continuous process of polymerization of a mixture of acrylate resin in a closed polymerization cell, formed by the two protective films themselves, made of a transparent material. Safety glass panels presently in commerce are constituted generally of two layers of glass bonded by an intermediate layer of a plastic material.
In order to be defined as a safety glass, a glass article of manufacture must have a large capability of absorbing shock energy, a high tear and splitting strength in order to prevent the rupture of the intermediate layer of plastic material by the glass splinters, a sufficient adhesion between the layers for minimizing the scattering and projection of glass splinters, in the event of a breakage, and a high optical quality, in order to satisfy the regulations presently in force for such articles of manufacture.
2. Description of the Prior Art
In the prior art, as an intermediate plastic material, a more or less thick film of plastified polyvinyl butyral is used, that in the following will be identified with the acronym PVB.
Such a film is produced with processes per se known and equally known and largely utilized is the technology for using it in the manufacture of safety laminated glass panels.
There is however a strong trend to substitute PVB with other polymeric films, in order to reach the objective of appreciable reduction of the manaufacturing costs for laminated glass panels. Presently, indeed, the cost of the PVB film amounts to about 30-40% of the production costs of a safety laminated glass panel.
In the prior art several attempts have been made to obtain films as an alternative to PVB, obtained from compositions of different materials. U.S. Pat. Nos. 3,509,015, 3,764,457, 3,900,446 and 4,241,140 report examples of compositions of mixtures for obtaining intermediate layers of transparent polyurethane material, suitable to be utilized in safety laminated glass panels.
Also polymers based on vinyl chloride have been proposed. U.S. Pat. No. 4,389,508 provides, as an example, an intermediate layer constituted of a thermoplastic resin resulting from the polymerization of vinyl chloride (80-98.5%) and a glycidylmethacrylate, suitably mixed with a plasticizer.
It is also known from the prior art that it is possible to use as intermediate layers acrylic polymers as taught by French Pat. No. 2,101,083. In this French patent, however, the technique for obtaining the intermediate layer of acrylic resin consists in casting between the glass panels to be laminated said acrylic resin in liquid form, that subsequently polymerizes in situ. The known art provides therefore useful suggestions about how to obtain alternative products to PVB presently utilized in the large industrial productions of safety laminated glass panels (in particular wind shields for cars and glaze unit for buildings), however it does not result from the prior art how to produce, on industrial scale, elastomeric films in substitution of PVB both in the dimension of a large industrial production, and in the possibility of applying such films in the lamination of safety glass panels, according to technologies similar to those utilized for PVB. As a matter of fact, such alternative films of the prior art appear either produced in quantities industrially not relevant, or they require in practice to modify the industrial equipments for the production of the laminated glass panels arranged for the use of PVB, with clear increases of costs.
In particular this is the case of the cited French Pat. No. 2101083, that, even teaching the use of acrylic polymers as intermediate layers, entails large modifications in the technology for industrial scale production of safety laminated glass panels.
Insofar as the production of PVB films is concerned, the prior art uses a heat polymerization process and a subsequent extrusion of the film. Moreover, in order to avoid the self-bonding of the coiled film, the same film must be dusted with talc or powders, or be maintained under an atmosphere with a defined relative humidity and at a temperature below 0 centigrade degrees. U.S. Pat. Nos. 3,872,197, 3,988,098, 4,211,742 and French Pat. Nos. 2214716 and 2294038 disclose a continuous polymerization for obtaining rigid acrylic sheets having a thickness equal or greater than 2 millimeters. The polymerization is performed by thermal way between two endless conveyor ribbons that operate as a polymerization cell. The product is a rigid sheet, that cannot be coiled and that cannot be utilized as intermediate layer for safety galss panels.